Miniature circuit breakers are widely used in engineering applications. This paper analysis the operation principle of overload protection and short circuit protection from its structure. Based on the actual situation in unit 3 and 4 of Qinshan Nuclear Power Plant, the basic reasons for several typical problems which have occurred on the miniature circuit breaker are analyzed and the treatment scheme is given respectively. Aiming at the defect that the breaker incorrect triped in main transformer cooling control-box in the high temperature environment, combined with the temperature coefficient correction and the parallel arrangement of the drop capacity effect, the miniature circuit breaker is rechosen to avoid the defect. In view of the defect that the upstream breaker of the PAMS cannot be switched on during the surge current, by giving the analysis of the match between the electromagnetic release characteristics of the miniature circuit breaker and the impact current of the downstream parallel load closing, the scheme is put forward to increase downstream parallel load breakers. According to the selectivity of the miniature circuit breaker, the unreasonable problem of the last stage distribution in the protection configuration is found out in view of the problem of RPR protecting the internal short circuit of the cabinet, and according to the requirements of the national standard, the selective test verification of the related model miniature circuit breaker is carried out. The reasonable part has been changed and carried out on the spot. After the actual operation of the unit, three typical problems occurred on the miniature circuit breakers have been effectively solved.

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